There has been a definite need for a source of highly coherent signals in the higher frequency regions, such as the infrared region and in the visible, so as to be available for wide bandwidth modulation, as a tunable local oscillatior, or as a spectroscopic source. Such a coherent source should be tunable over a wide bandwidth so that a large wavelength range can be searched.
Lasers, of course, are operative in the infrared and visible regions of the spectrum, but unfortunately, lasers have the characteristic of being tunable over only a narrow bandwidth. A klystron on the other hand is known to have a wide tuning range and a high degreee of coherence, but its upper frequency limit extends only to the submillimeter range.
When attempts are made to impress large information bandwidths directly on a laser carrier, problems arise because of the limited bandwidth capacity of the laser, and because wideband video modulation requires large driver power.
As will be discussed at length herein, we have provided a source of highly coherent signals in the higher frequency regions, so as to be available for a variety of purposes, and by the use of our novel techniques, large bandwidth capacities are available, being limited only by the microwave source that is used as the subcarrier for video information. The driver power requirement is reduced because of the fact that the modulation is impressed on such a subcarrier.
We accomplish the goal of providing the wideband tunable coherent source by combining a laser signal and a microwave signal in a nonlinear crystal to produce sum and difference frequency signals in the infrared or visible spectral regions, which signals are of course tunable over the frequency range of the microwave source. However, we encountered four major problem areas in achieving the goal. First, we had to find a material with the proper nonlinear coefficient, transmission, and orientation. Second, we had to arrange laser and microwave signals to enter such nonlinear material in aligned relationship, and phasematch the signals in the material to maximize the interaction in the material. Third, we had to separate the frequency that was generated from the laser signal that was used to generate the coherent, tunable wideband signal so that the tunable signal could be used without background interference from the laser signal, and fourth, we had to devise a sensitive detection scheme to verify the fact that the sum or difference frequency was being generated, and also so that the coherent tunable signal would be useful in system applications.
For an embodiment involving the use of a CO.sub.2 laser and a millimeter wave klystron, a gallium arsenide crystal was used as the nonlinear material. This crystal had a reasonable nonlinear coefficient, is transparent to both the laser signal and the millimeter wave signal, and was chosen and utilized so as to have an orientation that would produce a nonlinear interaction.
The alignment of the CO.sub.2 laser signal and the millimeter wave signal was accomplished by feeding the millimeter wave signal into a directional coupler and then into a waveguide where the gallium arsenide was located. The waveguide was designed so that phasematching between the laser signal and the millimeter wave signal could be achieved with the available crystal. The CO.sub.2 laser was then pointed through the straight section of directional coupler into the gallium arsenide loaded wave guide. Because of the intensity of the CO.sub.2 laser, it was possible to determine the location of the beam by burning a piece of paper before and after the loaded waveguide. The CO.sub.2 laser position was then adjusted so that the radiation was coming through the center of the waveguide.
The separation of the tunable signal from the CO.sub.2 laser signal was accomplished with a diffraction grating which separated the signals by virtue of the fact that it diffracted them at different angles. Approximately one meter away a small aperture was used to pass only the tunable signal.
The detection scheme we used to detect the tunable signal was a superheterodyne receiver, consisting of a mercury - cadmium - telluride infrared detector and a CO.sub.2 laser local oscillator operating on a transition that was adjacent to the transition on which the CO.sub.2 laser used for mixing was operated. In this way even very weak signals were able to be detected. A less sensitive receiver can be used if the signal level is increased, and in accordance with latter scheme, the klystron signal is chopped with a rotating wheel. A synchronous receiver with the same infrared detector and a Princeton Applied Research lock-in-amplifier can be used to envelope detect the signal, with the reference signal for the lock-in amplifier being derived from the chopping wheel.